The present invention relates to an optical type distance measuring device using image sensors, and more particularly to a device which continuously measures a distance from a first vehicle to another vehicle running ahead of the first vehicle (hereinafter referred to as "an inter-vehicle distance measuring device").
Conventional optical type distance measuring devices using image sensors are disclosed in Japanese Patent Publication Nos. Sho. 63-38085, 63-46363. Any of those devices, as shown in FIG. 10, contain right and left optical systems including lenses 1' and 2', respectively, which are spaced from each other by a base width L. Image sensors 3' and 4' are positioned at the focal distance f from the lenses 1' and 2', respectively. A signal processor 51 calculates a distance to an object 52 from the vehicle carrying the distance measuring device (referred to as the measuring-device carrying vehicle) by processing image signals received from the image sensors 3' and 4'.
Next, the method of measuring the distance up to the object 52 will be described. The signal processor 51 receives the image signals from the image sensors 3' and 4', which represent images of the object 52 formed on the image sensors 3' and 4' through the lenses 1' and 2', respectively. The processor then electrically superposes those image signals from the image sensors 3' and 4' by shifting them, and calculates, by the following equation, a distance R from the measuring-device carrying vehicle to the object 52 by the principle of the trigonometric measurement on the basis of an amount (d) of shifting when the image signals are most coincident with each other.
[Formula 1] PA1 R=(f.times.L)/d
On the other hand, a method for tracing the vehicle running ahead picked up by the image sensors, for example, is disclosed in Japanese Patent Publication No. Sho. 60-33352. A feature of the apparatus thereof is recited to trace an image of a target on the screen of a display unit, in which an operator manually sets a tracing gate (window) enclosing the target on the screen, while watching the screen.
The conventional distance measuring device is constructed such that the distance from the first vehicle to the object is obtained by comparing the images picked up by the right and left optical systems. Therefore, when measuring the inter-vehicle distance to the vehicle running ahead while the first vehicle carries the distance measuring device and where other vehicles are operating on the right or left side of the vehicle running ahead in the same direction, there is a problem that the operator of the first vehicle cannot detect which of the vehicles is under measurement.
Further when starting to trace the image of the vehicle running ahead, the operator manually sets a window for tracing an image on the display screen by means of a joystick or a trace pole while watching the screen. At this time, the operator's attention tends to be distracted from the forward direction, which may result in the occurrence of a traffic accident.